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‘Herd immunity’ target for ending pandemic could be lower than first thought

The target for “herd immunity” necessary to stop the coronavirus pandemic may be lower than previous estimates. (Source: Adobe Stock)

Achieving herd immunity – the point in the ongoing pandemic at which enough of a given population has been exposed to the virus to halt its spread – is a goal that has been frequently debated over the past several months. 

After crunching the numbers for herd immunity using updated rates of COVID-19 transmission within different population subgroups, mathematicians in Europe have provided a new general estimate for herd immunity: 43%. Their model suggests that once 43% of the people in a given population have contracted the virus, the outbreak would essentially be over.

If their estimate is accurate, populations could potentially reach herd immunity sooner, and with fewer deaths, than some forecasts early in the pandemic had suggested. While emphasizing that estimates of herd immunity will be different for every country, they said their research shows that social activity is a more important variable than age in reaching the herd immunity threshold.

To create their model, the mathematicians tweaked traditional calculations for herd immunity, adding two key factors which play an important role in the spread of infection. The first involved dividing the population into six age groups, each with a different average number of social contacts; the second involved subdividing these age categories into high, medium and low levels of social activity.

They also assumed that, on average, a person starts transmitting the virus to others three days after becoming infected and continues to spread it for four days, transmitting it to an average of 2.5 other people. The research was published in Science.

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